It is known that some internal combustion engines are equipped with an SCR system to reduce the emissions of nitrogen oxides (NOx). The SCR system generally includes a catalyst disposed in an exhaust pipe of the engine to convert the nitrogen oxides into diatonic nitrogen and water with the aid of a reducing agent. The reducing agent may be provided by an injector, which is located in the exhaust pipe upstream of the catalyst. Due to its position, the injector may become extremely hot and needs to be adequately cooled down. One of the solution currently adopted for performing this task is that of deviating part of the engine coolant into an auxiliary circuit having at least one portion in heat exchange relation with the injector. An example of this solution is described in the patent publication EP2192280.
A side effect of this solution is that the coolant pump, which delivers the engine coolant into the auxiliary circuit, is always active and arranged for generating a mass flow rate of coolant which is independent from the real cooling needs of the injector. In order to always guarantee a proper cooling of the injector, this mass flow rate takes into account the worst thermal conditions under which the injector may operate. However this approach implies that, when the injector is not operating under the worst thermal conditions, the power consumed by the coolant pump may become unnecessarily high, thereby causing a considerable impact on the fuel consumption.